None
Not Applicable
The present invention is related to the field of ion-sensitive sensors, and in particular ion-sensitive sensors employing field-effect transistors (FETs) as sensing devices.
It is known to build pH sensors using ion-sensitive field-effect transistors (ISFETs). In such sensors, a FET is placed in contact with an electrolyte solution, which causes the conduction characteristics of the FET to change in a manner that depends on the pH or other ion concentration of the solution. The FET""s conduction of current is detected by external circuitry which is calibrated in a manner such that the output of the circuitry is a digital or analog value representing the pH of the solution. This output can be used to drive a display or other processing circuitry as desired.
In general, prior ISFET sensors have been constructed such that the electrolyte solution contacts the FET in an area in which various circuit features of the FET are formed. These features include, for example, source and drain diffusions in the silicon substrate of the FET, and metal contacts that are used to provide electrical interconnection to the FET. It is not desirable for an electrolyte solution to contact these features, however, because improper operation or failure of the sensor can result. Thus prior ISFET sensors have generally employed some type of encapsulant to seal the FET from the electrolyte solution. Encapsulants are prone to leakage resulting from age-induced wear or other causes, and thus can contribute significantly to the failure of the ISFET sensors in which they are used.
It would be desirable to reduce or eliminate failure of ISFET sensors that arises from the use of encapsulants.
In accordance with the present invention, an ion-sensitive sensor and a method of fabricating an ion-sensitive sensor are disclosed. The sensor employs a silicon-on-insulator (SOI) ISFET, and maintains the required sealing of source and drain contacts without requiring encapsulants. The sensor fabrication method is compatible with existing SOI processing, so that the sensor can be readily integrated with other CMOS circuitry to enhance the performance and packaging of systems in which the sensor is used.
The disclosed sensor has an active layer of silicon with source and drain diffusion regions of a field-effect transistor formed therein, patterned layers of silicon oxide and metal on one side of the active silicon layer, and a layer of insulative support material on the metal and oxide layers. A continuous layer of silicon oxide on the other side of the active silicon layer has an exposed surface in the region of the field-effect transistor to enable the formation of surface charge in the exposed area of the continuous silicon oxide layer when placed in contact with an electrolyte solution. The surface charge induces a conductive channel in the channel region of the FET between the source and drain regions, enabling the flow of current between source and drain contacts under proper bias conditions.
Because the ion-sensitive surface is on the oxide-protected backside of the FET, there is no requirement for an encapsulant to protect the topside circuit features. Further, because of its structure the sensor can be easily integrated with other CMOS circuitry, which enables the electrical and packaging aspects of a sensor system to be improved.
In the disclosed sensor fabricating method, a field-effect transistor is formed on an active silicon layer of a silicon-on-insulator (SOI) wafer, and then an insulative support layer is formed over the active silicon layer. An exemplary support layer is polyimide. The substrate silicon of the SOI wafer is then removed, for example by chemical or mechanical means, to expose the buried silicon oxide layer of the SOI wafer. The exposed oxide layer may itself form the ion-sensitive surface of the final sensor, or alternatively additional treatments may be used to modify the sensing surface.
This fabrication method results in sensors whose topside contacts and components need not be exposed to an ion solution for proper sensor operation, so that no subsequent encapsulation process is required. Additionally, the process is compatible with existing SOI wafer processing, so that sensors and other CMOS circuitry can be formed on the same SOI wafer, improving both electrical and packaging aspects of the system in which the resulting sensor is used.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description which follows.